In the traditional cable television system, video signals from satellites and television stations are received at a control center ("head end"). The head end combines video signals from various sources and sends the signals to regional hubs for distribution to local nodes and individual cable subscribers.
The links between the head end and the regional nodes ("supertrunks") must carry 30 to 100 or more video channels from a cable system head end to points many kilometers away. In the early CATV systems, coaxial cable and microwave towers were used for supertrunks to cover such distances. However, coaxial cable and microwave links tended to be somewhat noisy. Further, coaxial cable for such lengths required many repeaters or amplifiers and the failure of one repeater or amplifier could cause interruption of cable service to numerous subscribers.
Optical fibers offered a solution to these problems and by the late 1980s, fibers had become a standard for CATV supertrunks. Since then, optical fiber systems have spread further into CATV distribution networks. With CATV optical fiber systems in place, a logical step in order to increase capacity is to multiplex the video signals. Time division, frequency division and other types of multiplexing have been successfully utilized in voice and data telecommunications.
Dense wavelength division multiplexing (WDM) is being rapidly deployed in high-speed digital networks, particularly in long-haul 2.5 and 10 Gbits/sec systems. In addition to providing greater capacity over a single link, WDM affords some tools for networking, such as separating different CATV service options by grouping them on different wavelengths. As in any analog transmission, the carrier-to-noise ratio (CNR) of the CATV transmission must be adequate in order to provide a high-quality signal to the cable subscriber. Therefore, in applying WDM to CATV transmission, it is important to minimize all sources of noise and crosstalk. Analog CATV fiber transmission systems could take advantage of WDM, both for upgrading capacity and network flexibility, but it appears that WDM analog CATV fiber systems suffer from large crosstalk due to optical fiber nonlinearity at even modest optical power levels. For example, a 1.3 .mu.m 2-wavelength dense WDM system showed -42 dBc of crosstalk with a launched power of +9 dBm/channel. (See K. Kikushima, H. Yoshinaga and M. Yamada, "Signal Crosstalk Due to Fiber Nonlinearity in Wavelength-Multiplexed SCM-AM-TV Transmission systems," Proceedings of the Conference on Optical Fiber Communication, 1995, paper PD-24.)
In another case, a similar system at 1.5 .mu.m showed -47 dBc of crosstalk at an identical launched power. See A. Li, C. J. Mahon, Z. Wang, G. Jocobsen and E. Bodtker, "Experimental Confirmation of Crosstalk Due to Stimulated Raman Scattering in WDM AM-VSB CATV Transmissions Systems," Electronics Letters, vol. 31, pp. 1538-1539 (1995). It appears that this crosstalk is due to Stimulated Raman Scattering (SRS), even though the launched power levels are much lower than those known to cause significant SRS degradation in digital systems. Since this crosstalk appears as a spurious carrier at the center of an RF channel when the desired carrier is turned off, it mimics a Composite Triple Beat (CTB) signal, and thus, needs to be suppressed to -60 to -65 dBc. If not suppressed to these levels, then this large, nonlinear crosstalk severely limits the usefulness of WDM in analog CATV fiber systems.